1. Field of the Invention
This invention relates generally to the field of semiconductor memories and specifically to sensing amplifiers therefor.
2. Prior Art
In semiconductor memory device design, the sensing amplifier has always been the most important and critical part of the design. Due to the large loading capacitance on the internal bit line and the associated external data bus, the charging and discharging time of the bit line has inherently been very slow, which limited the sensing speed of the static memory when a conventional differential amplifier and/or a common source amplifier in the front stage of the sensing amplifier was used. The former has the difficulties of first having an output swing from the differential amplifier that is very limited with the currently used five volt power supply, and secondly, it takes a relatively large input signal swing which means longer charging and discharging time of the bit line.
In the latter case, the common source amplifier can be made to be cascaded in stages with feedback thus allowing relatively fast sensing, but it is prone to having an instability problem due to a high open loop gain and a relatively long feedback path. In addition, it is relatively sensitive to fabrication processing variation which becomes critical as ROM density approach VLSI.